A seat has a seat base or cushion, and often a seat back. The seat back allows a user to lean back, and the seat back supports the user's back as the user leans back. The seat back therefore needs rigidity to resist the force of the user leaning back. It is often desirable for the seat back to have cushioning in order to provide comfort to the user.
When producing a seat back, a back frame of rigid materials is often provided to provide the support for the user. When choosing a frame, strength is a significant consideration. The materials of the frame must be chosen and arranged so that the seat back can properly support the user. Cost is often a significant consideration, both in the cost of the raw materials, and in the cost of assembly. When the seat back is for the seat of a vehicle, the size and weight of the seat back are also often a significant consideration. It is often desirable to have the size of the frame be as small as possible to provide more space in the vehicle for other purposes/features. It is also often desirable to have the weight of the frame be as low as possible in order to make the performance of the vehicle better. Strength, material cost, size, ease of assemble and weight often are in conflict because optimizing one of these will cause one or more of the others to be non-optimal. Therefore, it is beneficial to provide a back frame which is beneficial in as many areas as possible, without having significantly poor characteristics in any one or more areas.
A typical rear seat back frame construction is shown in FIGS. 3-5. In the prior art embodiment of FIG. 5, the axial ends of right and left side member tubes 14 are connect to respective axial ends of upper and lower cross-member tubes 20 by respective external corner brackets 24. In the prior art embodiment of FIGS. 3-4, the axial ends of right and left side-member tubes 14 are interlocked into an opening 22 at each axial end of upper and lower cross member tubes 20 with all cross member tubes 20 and right and left side member tubes 14 being of the same dimensions. The typical back frame includes added recliner attachment brackets 26, to attach recliner upper plates to the back frame.